Food Webs

A food web is what you get when you combine many food chains in one ecosystem. Real ecosystems do not have neat single chains; almost every plant has dozens of animals that eat it, and almost every predator has more than one kind of prey. When you draw all the eat-and-be-eaten arrows for one ecosystem, you get a tangled network: a web of crisscrossing food chains, with all the species linked together in a complex pattern. Food webs are the most realistic picture of how ecosystems actually work.

  • Made ofMany food chains combinedIn one ecosystem
  • Typical complexity50 to 200+ speciesIn a small ecosystem
  • Most realistic ofReal ecosystemsSingle food chains are oversimplified
  • First detailed web1923Charles Elton, Bear Island in the Arctic
  • Most studied webYellowstone wolf webAfter 1995 wolf reintroduction
  • Removing one speciesCan change the whole webEspecially keystone species

Why ecosystems are webs, not chains

A single food chain might look like:

grass → rabbit → fox

But in real life, grass is eaten by rabbits AND deer AND mice AND grasshoppers AND many other creatures. Rabbits are eaten by foxes AND owls AND buzzards AND wildcats AND snakes. Foxes also eat mice, voles, beetles, fruit and the occasional discarded sandwich. None of these can be drawn as a single line. Connect them all up and you get a web.

Building a food web

To build a food web for an ecosystem, ecologists list every species in the area and then add an arrow between every "X is eaten by Y" pair they can find. Even a small ecosystem like a pond can have hundreds of arrows. Most real food webs include:

  • Many producers (different kinds of plant or algae).
  • Many primary consumers (herbivores) each eating several producers.
  • Many predators, each eating several different prey.
  • Omnivores: animals that eat both plants and other animals, spanning trophic levels (humans, bears, raccoons).
  • Decomposers at every level, breaking down dead matter and feeding back into the soil.

Why webs are more stable than chains

One big advantage of a web over a single chain is stability. If a single predator suddenly disappears, the prey species can shift to being eaten by other predators in the web. If a particular plant fails one season, herbivores can switch to others. Food webs absorb small shocks more easily than single chains can.

Generally, the more complex a food web is (more species, more connections), the more resilient the ecosystem is to disturbance. This is one of the strongest scientific reasons for protecting biodiversity: complex ecosystems with lots of species are more likely to survive and bounce back from disasters than simple ones.

Keystone species and food webs

Some species sit at central positions in a food web, with many other species depending on them. These are called keystone species (after the central wedge that holds up an old stone arch). Removing a keystone species can cause the whole web to collapse.

The most famous example is the sea otter in the kelp forests of the North Pacific. Sea otters eat sea urchins. Sea urchins eat kelp. When otters were hunted almost to extinction for their fur in the 1700s and 1800s, urchin numbers exploded, and they grazed the kelp forests down to bare rock. With the kelp gone, hundreds of other species that depended on it (fish, shellfish, sea birds) also disappeared. When sea otter populations were protected and recovered in the 20th century, the kelp forests grew back and so did the rest of the web. A single species reshaped an entire ocean ecosystem.

Fact The first detailed food web was published by British ecologist Charles Elton in 1923, after he spent the summer of 1921 studying the wildlife of Bear Island in the Norwegian Arctic. Elton drew arrows between every species that ate each other, and the resulting diagram (with plankton at the bottom and Arctic foxes at the top) became one of the most influential pictures in the history of ecology. Almost every food web textbook diagram still follows his basic design.

Drawing a food web yourself

To draw a simple food web for a back garden, you could include:

  • Producers: grass, dandelions, vegetable plants, fruit bushes.
  • Primary consumers: caterpillars, slugs, snails, aphids, rabbits, mice.
  • Secondary consumers: garden birds, frogs, ladybirds (which eat aphids).
  • Tertiary consumers: sparrowhawks, owls, foxes.
  • Decomposers: earthworms, woodlice, fungi, bacteria.

Even a small garden can easily contain over 50 species with dozens of feeding connections. A back garden may be small, but its food web can be surprisingly rich.

Did you know? One of the most complex food webs on Earth is in the Antarctic Ocean, but it has surprisingly few species at the top. The whole web rests on krill, tiny shrimp-like crustaceans that are eaten by everything from small fish to giant blue whales. Estimates put the total weight of Antarctic krill at over 500 million tonnes, possibly more than any other single species on the planet. Without krill, almost every Antarctic mammal, bird and fish would collapse.
Deeper dive: trophic cascades and the rewilding movement

A trophic cascade is when a change at one level of a food web ripples up and down through other levels. The classic example is when adding (or removing) a top predator changes which plants grow in an ecosystem, even though predators and plants seem far apart in the food chain.

The Yellowstone wolves are the most famous case. When wolves were reintroduced to Yellowstone in 1995 after a 70-year absence, they killed and frightened the local elk, which had been over-grazing the river valleys. With fewer elk eating willow and aspen seedlings, young trees finally got to grow up. With trees back along the rivers, beavers returned and built dams, which slowed the rivers, created new ponds, and provided habitat for fish, frogs, ducks and many other species. Even the courses of the rivers themselves shifted as plant roots stabilised the banks. All of this came from the return of a single predator.

This kind of trophic cascade is now the basis of a worldwide conservation movement called rewilding: trying to restore healthy ecosystems by bringing back key species, especially predators, that have been lost. Wolves have been reintroduced to parts of Europe. Beavers (themselves an ecosystem engineer) are being released into British rivers. Various other reintroduction projects (lynx, sea otters, even cheetahs) are running around the world.

Rewilding shows the deep practical value of understanding food webs. By thinking about ecosystems as connected networks rather than separate species, conservationists can do much more good with much less effort: bring back the right keystone species, and nature does the rest of the work.

For the simpler picture, see food chains. For who plays which role, see producers, consumers and decomposers.